How Big a Deal Is Trump’s Fuel Economy Rollback? For the Climate, Maybe the Biggest Yet

Trucks headed for delivery in California. Americans have favored larger vehicles for years now.CreditMike Blake/Reuters

WASHINGTON — President Trump’s proposal this week to weaken fuel-efficiency standards for cars and light trucks could be his most consequential climate-policy rollback yet, increasing greenhouse gas emissions in the United States by an amount greater than many midsize countries put out in a year.

Assuming the plan is finalized and survives legal challenges, America’s cars and trucks would emit an extra 321 million to 931 million metric tons of carbon dioxide into the atmosphere between now and 2035 as a result of the weaker rules, according to an analysis by the research firm Rhodium Group. A separate estimate by the think tank Energy Innovation pegged the number even higher, at 1.25 billion metric tons.

To put that in context, the extra pollution in 2035 alone would be more than the current annual emissions from countries like Austria, Bangladesh or Greece, the Rhodium Group analysis found.

How big a deal is that for global warming? The Trump administration claims it is negligible. By 2100, officials argued in their proposal, concentrations of carbon dioxide in the atmosphere would only be 0.65 parts per million higher under the rollback than they would be if the stricter Obama-era rules had stayed in place. (Current levels in the atmosphere are around 410 parts per million.)

But that’s the wrong way to look at it, according to Trevor Houser, lead author of the Rhodium Group report. Any single climate policy from a single country will look relatively modest in isolation. Stopping global warming will require a wide variety of efforts to cut emissions from every sector of nearly every country. “In that context, this single policy really does have a big impact,” he said.

His analysis estimated that the fuel-economy rollback could have a bigger effect on emissions than either Mr. Trump’s attempts to repeal the Clean Power Plan — a federal rule to curb pollution from coal-fired power plants — or his efforts to scale back regulations on oil and gas operations that release methane, a potent greenhouse gas, into the atmosphere.

But pollution from cars and trucks has proved much trickier for states to take on. Transportation now accounts for one-third of America’s carbon-dioxide emissions, surpassing power plants as the largest source, and vehicle emissions have been steadily rising over the past few years. Federal fuel-economy standards were widely seen as a vital tool for curbing gasoline use.

“We’ve seen nowhere near the same progress in transportation as we’ve seen in electricity,” said Jordan Stutt, a policy analyst at the Acadia Center, a group in New England that is pushing for cleaner energy.

The original Obama-era standards would have required automakers to roughly double the fuel economy of their new cars, pickup trucks and S.U.V.s by 2025, putting out vehicles that would average roughly 36 miles per gallon on the road. The Trump proposal would halt the rise of those standards after 2021, when new cars were expected to average around 30 miles per gallon.

The Obama-era rules also granted California permission to set up a separate, more ambitious program to mandate more zero-emission cars on the road. Nine other states in the Northeast have adopted that program, which would require roughly 8 percent of new vehicles sold in-state to be plug-in hybrid, electric or hydrogen fuel cell models.

The Trump proposal plans to challenge California’s authority to mandate zero-emissions carsand to halt the clean vehicle program, which could dramatically slow the adoption of electric vehicles around the country in the near term.

“The zero-emissions vehicle waiver has been the biggest catalyst to date in bringing electric vehicles to market,” said Don Anair, research and deputy director of the Clean Vehicles Program at the Union of Concerned Scientists.

There are, however, a few important factors that could potentially counteract the climate impact of the Trump administration’s rollback, assuming that it survives any court challenge by California and other states and becomes final.

First, fuel prices will matter enormously. If oil prices increase significantly over the next decade, then many drivers might opt to buy more efficient vehicles regardless of what federal standards require. (The lower emissions numbers in the Rhodium Group analysis are based on a scenario where oil prices are high.)

But if gasoline prices stay at current levels — around $2.80 per gallon — or drop further, then Americans are expected to continue to buy S.U.V.s and other gas guzzlers, as they have been doing in increasing numbers the past few years.

Second, states could try to enact other fresh policies to try to cut emissions from the transportation sector and blunt the impact from Trump’s rollback. California and New York, for instance, have been offering tax breaks for people to buy electric vehicles, and they have been investing hundreds of millions of dollars in new charging infrastructure.

Other Northeastern states have been participating in discussions on how to reduce vehicle emissions, through steps like expanding mass transit, buying electric buses or reconfiguring cities to make them denser and more walkable.

But some of these state policies can be politically difficult and take time to enact. In the absence of stricter federal fuel economy standards, states like Connecticut and Maryland that have set legislative targets for reducing economywide emissions might struggle to meet their goals.

“Transportation is extremely complicated and it really takes all levels of government working together,” said Vicki Arroyo, the executive director of the Georgetown Climate Center, who has been working with states on plans to cut emissions from transportation. If the federal government pulls back, she said, “it’s a tremendous setback.”

The automakers themselves are another wild card. While many manufacturers have been developing new electric car models in response to the ever-rising fuel economy standards, it’s not clear how many would completely pull back if the standards were frozen. China and Europe are continuing to push hard on fuel efficiency and battery-powered vehicles, and automakers have those international markets to consider.

And the biggest wild card of all? What the next president might do. “If a new administration came in, they’d have a blank slate for rethinking the standards entirely, and there are a lot of ideas out there for standards that would be even more effective” than the Obama-era rules, said Mr. Houser.

If a future president ultimately managed to put even stricter vehicle rules in place, he said, “that would certainly reduce the magnitude of the emissions impact that we’re projecting.”

Brad Plumer is a reporter covering climate change, energy policy and other environmental issues for The Times's climate team. @bradplumer

A version of this article appears in print on , on Page A14 of the New York edition with the headline: How Much Car Pollution? A Whole Country’s Worth. Order Reprints | Today’s Paper | Subscribe